I do not have anything set up other than factory defaults. Take a clean blade and install ESXi 5.1. In the management network settings, connect both NICs (5709S), DHCP, reboot, and all 0's.
Do the same with a clean blade using 5.0 and all works. I do not know what VMware changed between 5.0 and 5.1 in respect to default NIC configurations for a standard vSwitch or what I need to undo.